This invention relates to measurement systems, and more particularly, to capacitance measurement systems.
Capacitive sensors have often been used to provide the input signals for many forms of prior art transducer instrumentation, for example, scales or balances, accelerometers, and pressure transducers or proximity gauges. In such systems, the precision at which measurements can be made is very much a function of the stability of the circuit interfacing the sensor portion and read-out-portion of the system. In general, the prior art systems have permitted relatively low precision measurements due to the sensitivity of the interface circuit to combinations of various factors such as drive signal waveshape, drive signal amplitude, drive signal frequency, and temperature dependent component parameter variation.
More particularly, the class of system exemplified by U.S. Pat. No. 3,318,153 to Lode includes circuit interface which generates an output signal as derived from a rectification and summing of current signals whose amplitude is dependent on both the magnitude and frequency of an applied drive signal, thereby requiring a high voltage drive signal at relatively low frequencies and establishing a relatively large power requirement. Thus, that system has a relatively high sensitivity to both drive signal amplitude and frequency, and, as a result, means is provided by Lode to maintain the amplitude-frequency product for the drive signal to be constant.
Other forms of prior art systems, such as that illustrated by U.S. Pat. No. 3,572,319 to Bittner are directed to systems for differential capacitance measurement which are drive signal waveshape sensitive as well as amplitude and frequency sensitive. More particularly, that system utilizes a means for extracting the average of the rectified voltage applied across the sensor, that average being a function of the waveshape of the driving signal.
Accordingly, it is an object of the present invention to provide a system for determining the capacitance associated with a sensor capacitor with the system being relatively independent of the amplitude, frequency, and waveshape of the driving signal.
Another object is to provide a signal linearly related to the capacitive reactance of a sensor capacitor.